Boil potatoes in salted water till cooked. Drain water and keep aside the potatoes.
(You may alternatively use new baby potatoes with skin. In that case wash and prick the baby potatoes before boiling.)
Dry roast the pickling spices- Mustard Seeds, Fennel, Carom seeds, Cumin seeds, Onion Seeds (Kalongi),fenugreek seeds for a minute and coarsely ground them. Keep aside.
Heat mustard oil in a wok and bring it to a smoking point. Reduce heat and add asafoetida and the grounded pickling spices.
Cook for few seconds and add chopped onions .Cook until onions are light brown.
Add dry spices-turmeric, red chilli powder, black salt and salt. Add vinegar and some water; cook for 2 minutes.
Add boiled potatoes and chopped green chillies, toss to mix well.
Finish with chopped coriander and give a final mix.
Remove in a bowl and serve or store in refrigerator.